A surface waveguide is a light-guiding element, much like an optical fiber, which is formed on the surface of a rigid substrate. Surface waveguides are used for many applications including telecommunications, chemical sensing, and force sensing.
A surface waveguide is characterized as having a central region or “core” and a surrounding “cladding.” An optical signal travels through a surface waveguide as an optical mode propagating through the core. The optical signal is substantially confined to the core by the cladding. The guiding property of a surface waveguide arises from a difference in the refractive index, n, between the core and the cladding. For a surface waveguide, the refractive index of the cladding is typically slightly lower than the refractive index of the core.
Surface waveguides can be fabricated in various forms including slab waveguides, ridge waveguides, and stripe waveguides. A slab waveguide comprises a planar thin film optical core sandwiched between two planar thin film claddings. The cladding above and below the core confine the propagating optical mode vertically, but not laterally.
A ridge waveguide is similar to a slab waveguide, but also includes a protruding ridge of material through which an optical mode propagates. The structure of the ridge substantially confines the mode both vertically and laterally, except where the ridge meets the slab. It is possible that a mode can exist in the slab area outside the ridge portion.
A stripe waveguide is essentially a ridge waveguide in which the slab portion has been etched away. The optical mode is confined to the stripe since there is no core material anywhere else.
Surface waveguide applications demand consistent optical and mechanical properties of the materials used in the waveguides. Surface waveguides have been formed from a variety of materials, including different types of glasses (e.g, silicon dioxide, boro-phosphosilicate glass, phosphosilicate glass, etc.), silicon nitrides, silicon oxy-nitrides, gallium arsenide, indium phosphide, silicon, and lithium niobate, or a combination thereof.
The surface waveguide is formed by successively depositing and appropriately patterning thin films of optical materials onto the surface of a substrate. Low pressure chemical vapor deposition (LPCVD) is a common method of forming the thin-film layers. In an LPCVD system, the glass is deposited onto the surface of a silicon wafer in high temperature furnaces into which different precursor gasses are injected, resulting in a chemical reaction that deposits glass on the surfaces of the silicon wafer.
Many LPCVD-deposited thin-films exhibit inherent residual stress and induced strain after deposition, primarily due to a mismatch of the coefficient of thermal expansion of the thin-film material and the substrate material. The magnitude of residual stress can increase with film thickness, which leads to higher induced strain. Above a certain “critical thickness,” which is unique for each material, the induced strain in the film exceeds the fracture strain. The film therefore fractures, usually as it cools to room temperature from the elevated deposition temperatures (e.g., typically in the range of 500 to 1000° C.). As a consequence of this behavior, there is a limit to the thickness to which many thin-film materials can be deposited.
Thin films that are formed from either stoichiometric silicon nitride (Si3N4) or TEOS (silicon dioxide deposited using tetraethylorthosilicate as a precursor gas), are highly desirable for use as waveguide cores due to their consistent properties and lack of light-scattering centers. Unfortunately, these materials are subject to the aforementioned critical-thickness constraint, thereby limiting their utility for use in surface waveguides. In particular, thin films formed from stoichiometric silicon nitride are limited to a thickness of about 350 nm and thin films formed from TEOS have critical thickness limitation of about 1850 nm. In many instances, it is desirable to have core cross sections that exceed these critical thicknesses in both dimensions (e.g., to facilitate coupling to optical fibers or active devices such as lasers or detectors, etc.).